ᐅ Sealing the foundation slab before the roof is installed

Even though I am already talking to myself, just to clarify: if he uses the appropriate material for the entire slab—the same material he would place under the masonry—would it then be technically correct to build the partition walls on top of the fully sealed concrete slab?

It has again been found non-weldable...
Under masonry, a sanded waterproofing membrane (e.g. G200DD) and on the slab a waterproofing membrane for welding (e.g. G200S4)

And how does he connect them then?
What do you see as the disadvantage if I build up the wall higher?
What could happen?

The waterproofing membrane is then welded over the overlapping areas.
It can happen that the masonry presses into the waterproofing membrane.
I would have no concerns with non-load-bearing walls.

Hmm, so it’s not as problematic as I thought.
I assumed it was completely wrong.
So if I let him do it that way, I don’t have to worry?

The only thing that still puzzles me is the timing—why does he want to waterproof now when the roof structure will only be installed in week 48?

Which is worse, water on the raw concrete slab or water on the waterproofing membrane?

The worst case is not both, but very bad would be water trapped between the waterproofing membrane and the concrete slab!